fix: Make symlink_ds.sh more robust by checking if /usr is a distinct mountpoint

[dylanmtaylor]

Pull request

Purpose
This handles installation on immutable filesystems better. If /usr is not a distinct mountpoint, the existing check logic does not work as intended

Approach
If /usr does not have its own distinct mountpoint, we use the immutability check on the root filesystem to determine if it is read only or not.

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